Solid state semiconductor lasers are important devices in applications such as optoelectronic communication systems and high-speed printing systems. Recently, there has been an increased interest in vertical cavity surface emitting lasers ("VCSEL's") although edge emitting lasers are currently used in the vast majority of applications. A reason for the interest in VCSEL's is that edge emitting lasers produce a beam with a large angular divergence, making efficient collection of the emitted beam more difficult. Furthermore, edge emitting lasers cannot be tested until the wafer is cleaved into individual devices, the edges of which form the mirror facets of each device. On the other hand, not only does the beam of a VCSEL have a small angular divergence, a VCSEL emits light normal to the surface of the wafer. In addition, since VCSEL's incorporate the mirrors monolithically in their design, they allow for on-wafer testing and the fabrication of one-dimensional or two-dimensional laser arrays.
A known technique to fabricate VCSEL's is by a lateral oxidation process, as schematically illustrated in FIGS. 1 and 2. Under this approach, a laser structure comprising a plurality of layers is formed upon substrate 10. These layers include an active layer 12 and an AlGaAs layer 14 with a high aluminum content. The AlGaAs layer 14 is placed either above or below the active layer of a laser structure. Then, the layered structure is masked and selectively etched to form a mesa structure 22 as illustrated in FIG. 2. As a result of the etching, the AlGaAs layer 14 with a high aluminum content adjacent to the active layer 12 is exposed at the edges of the mesa structure 22. To form the lasing emissive region or "aperture", this AlGaAs layer is oxidized laterally from the edges towards the center of the mesa structure as represented by arrows A. Other layers in the structure remain essentially unoxidized since their aluminum content is lower. Consequently, their oxidation rates are also substantially lower. Therefore, only the AlGaAs layer with high aluminum content is being oxidized. The oxidized portions of the high aluminum content layer become electrically non-conductive as a result of the oxidation process. The remaining unoxidized region, which is conductive, in the AlGaAs layer forms the so-called "aperture", a region which determines the current path in the laser structure, and thereby determines the region of laser emission. A VCSEL formed by such a technique is discussed in "Selectively Oxidized Vertical Cavity Surface Emitting Lasers With 50% Power Conversion Efficiency," Electronics Letters, vol. 31, pp.208-209 (1995).
The current lateral oxidation approach has several disadvantages, such as large mesa, large oxidation region, and poor control of the aperture size. A key disadvantage of this approach is the difficulty in controlling the amount of oxidation. Generally, the desired device aperture is on the order of one to ten microns (.mu.m), which means that several tens of microns of lateral oxidation will typically be required in order to fabricate the device when oxidizing in from the sides of the much larger mesa, which must typically be 50 to 100 microns in size. Since the size of the resulting aperture is small relative to the extent of the lateral oxidation regions, the devices formed generally have severe variations in aperture size as a result of non-uniform oxidation rates from wafer to wafer and across a particular wafer. The oxidation rate of AlGaAs depends strongly on its aluminum composition. Any composition non-uniformity will be reflected by changes in the oxidation rate, which in turn creates uncertainty in the amount of oxidation. The process is also relatively temperature-sensitive. As the oxidation rate varies, it is difficult to ascertain the extent to which a laser structure will be oxidized, thereby decreasing reproducibility in device performance. In short, such a process often creates various manufacturability and yield problems.
Another disadvantage of a VCSEL formed by a traditional lateral oxidation approach is the difficulty it creates in forming high density laser arrays. In order to oxidize a buried layer of high aluminum content, an etching process is performed leaving a mesa. After the etching of this mesa, lateral oxidation is performed such that the oxidized regions define a laser aperture of a particular size. The use of a mesa structure, in part, limits the minimum spacing between two lasers in an array. The step height of the mesa is typically several microns because of the need to etch through a thick upper DBR mirror. Additionally, the top surface of the mesa also has to be relatively large so that a metal contact can be formed on it without covering the lasing aperture. Typically, the minimum size of an electrical contact is approximately 50.times.50 .mu.m.sup.2. Hence, the step height of the mesa and the placement of the electrical contact on the surface make it difficult to form highly compact or high density laser arrays.
A solution to some of the problems associated with a typical mesa structure is the use of a shallow mesa. In order to use a shallow mesa, the upper mirror is not formed by an epitaxial process. Instead, the upper mirror is formed by a deposited multilayer dielectric material, which reflects light. Electrical contact is made directly onto the upper portion of the active region. Devices formed under this approach have been fabricated on mesas with widths of approximately twelve microns. However, the added complexity of depositing a dielectric material and using a liftoff process to define the contact make it difficult to optimize the devices for low threshold current and high efficiency.
A VCSEL formed by a traditional lateral oxidation approach often suffers from poor mechanical or structural integrity. It is well-known that the upward pressure applied during a packaging process may cause delamination of the entire mesa since the bonding of the oxide layer to the unoxidized GaAs or AlGaAs is generally weak.
Light from typical VCSELs is usually polarized along one of two orthogonal directions along the wafer surface. The dominant polarization can switch back and forth between these two orthogonal orientations as the operating current to the VCSEL is varied because there is no natural preference for either orthogonal direction. The polarization instability is a major drawback because it limits VCSELs to applications where no polarization sensitive optical elements are present. Moreover, if the VCSEL is modulated, sudden changes in polarization states can result in undesirable light intensity fluctuations that contribute to signal noise.
There are several known methods for controlling VCSEL polarization. These include making devices with anisotropic mesa geometries as described by K. Choquette and R. Leibenguth in "Control of vertical-cavity laser polarization with anisotropic cavity geometries", IEEE Photonics Technology Letters, vol. 6, no. 1, pp. 40-42, 1994, making devices with tilted etched-pillar structures as described by H. Y. Chu et al. in "Polarization characteristics of index-guided surface emitting lasers with tilted pillar structure", IEEE Photonics Technology Letters, vol. 9, no. 8, pp. 1066-1068, 1997, use of dielectric top mirrors with coated sidewalls as described by M. Shimuzi et al. in "Polarisation control for surface emitting lasers", Electronics Letters, vol. 27, no. 12, pp. 1067-1069, 1991, using substrates having a misoriented surface as described in Compound Semiconductor, May/June, p. 18, 1997 or milling a cavity next to a completed gain-guided device as described by P. Dowd et al. in "Complete polarisation control of GaAs gain-guided top-surface emitting vertical cavity lasers", Electronic Letters, vol. 33, no. 15, pp. 1315-1317, 1997.